The present invention relates to connections between a bore connector or copper and field windings and exciter rings at opposite ends of the bore connector in dynamoelectric machines and particularly relates to terminal studs having powder coated electrical insulation.
The rotors of dynamoelectric machines typically comprise relatively large-diameter cylindrical bodies containing field windings for producing magnetic flux which, in turn, produces stator current and voltage. These field windings are normally carried in a series of longitudinal slots along the outer circumference and extend the length of the rotor body, terminating in end turns. The rotor is also provided with a bore connector, also known as xe2x80x9cbore copperxe2x80x9d (insulated conductors embedded in small-diameter shafts that extend from opposite ends of the rotor body for electrical connection with the exciter/rectifier assembly). The bore connectors make electrical connection at opposite ends with the field winding and the exciter brushes.
To make electrical connection between the field windings and the inner end of a bore connector, a main terminal is inserted into a radial bore of the rotor shaft. The main terminal has tapered threads at its radial inner end for engaging female tapered threads in the bore connector. The opposite end of the main terminal is electrically coupled via flexible leaves to the field winding. At the opposite end of the bore connector, a collector terminal stud electrically interconnects the bore connector and the exciter brushes. Conventionally, the terminal studs are electrically insulated from the rotor by hand-wound epoxy glass cloth composite insulation. Manual application of the insulation is costly and inconsistent. Additionally, in hydrogen-cooled generators, it is necessary that the collector terminal stud serve also as a mechanical seal to contain the coolant hydrogen gas within the generator. The cloth insulation has a permeability to hydrogen and is thus not an ideal material with which to form a seal. Accordingly, there is a need for terminal studs in electrical generators which have improved dielectric properties, as well as sealing capacity.
In accordance with a preferred embodiment of the present invention, the terminal studs of a generator are coated with a powdered electrical insulating material. The powder coated insulation has equal or better dielectric strength than the prior cloth electrical insulation and is less hydrogen-permeable in those applications where the terminal stud is required to serve both as an electrical insulator and a seal, i.e., at the collector terminal stud. The powdered insulation may comprise an epoxy material which is commercially offered by the 3M Company under the tradename Epoxy 3M 5230. It has been found that terminal studs coated with the powder insulation have improved properties, as outlined below.
The powder insulation may be applied by a number of different methods, including employing an electrostatic fluid bed using an electrostatic charge to contact the powder particles with the terminal stud, a spray gun to spray electrostatically charged particles onto the terminal stud or a fluidized bed where the part is preheated and dipped into the bed to bond the particles to the stud. With respect to the collector terminal stud, a gasket is compressed by a threaded nut to engage radially about the powder coated insulation on the collector stud to seal thereagainst. Because the powder insulation is smooth, the gasket is more fully compliant against the powder, forming a more effective seal upon comparison with the seal formed using cloth composite insulation as in the past.
In a preferred embodiment according to the present invention, there is provided in a dynamoelectric machine having a rotor, a radial bore in the rotor, a bore connector extending generally in an axial direction along a portion of the rotor and having a generally radially outwardly opening threaded aperture, a stud in the radial bore for electrically coupling the bore connector and electrical contacts carried by the rotor, the stud having a threaded connection at one end for threaded engagement with the bore connector in the threaded aperture thereof and an electrical insulator about a portion of the stud for electrically insulating the stud and the rotor from one another, the insulator including a thermoset or thermoplastic powder resin coating on the insulator portion.
In a further preferred embodiment according to the present invention, there is provided in a dynamoelectric machine having a rotor, a radial bore in the rotor, a bore connector extending generally in an axial direction along a portion of the rotor and having a generally radially outwardly opening threaded aperture, a stud in the radial bore for electrically coupling the bore connector and electrical contacts carried by the rotor, the stud having a threaded connection at one end for threaded engagement with the bore connector in the threaded aperture thereof, a method of forming the stud with an electrical insulator about a portion of the stud comprising the steps of (a) providing an electrostatic fluid bed containing a thermoset or thermoplastic resin powder, (b) passing air through the powder in the bed to obtain a rolling boil of the powder, (c) applying an electrostatic potential to the powder in the bed, (d) grounding the stud enabling the powder to be attracted to and forming a coating on the stud and (e) curing the powder after application to the stud.
In a further preferred embodiment according to the present invention, there is provided in a dynamoelectric machine having a rotor, a radial bore in the rotor, a bore connector extending generally in an axial direction along a portion of the rotor and having a generally radially outwardly opening threaded aperture, a stud in the radial bore for electrically coupling the bore connector and electrical contacts carried by the rotor, the stud having a threaded connection at one end for threaded engagement with the bore connector in the threaded aperture thereof, a method of applying powder resin to the stud comprising the steps of (a) spraying electrostatically charged thermoset or thermoplastic resin powder onto the stud and (b) grounding the stud to cause the sprayed powder resin to adhere to the surface of the stud and curing the resin on the stud.
In a further preferred embodiment according to the present invention, there is provided in a dynamoelectric machine having a rotor, a radial bore in the rotor, a bore connector extending generally in an axial direction along a portion of the rotor and having a generally radially outwardly opening threaded aperture, a stud in the radial bore for electrically coupling the bore connector and electrical contacts carried by the rotor, the stud having a threaded connection at one end for threaded engagement with the bore connector in the threaded aperture thereof, a method of applying a resin powder to a stud comprising the steps of (a) providing a fluidized bed containing thermoset or thermoplastic powder, (b) fluidizing the powder in the bed to a rolling boil, (c) preheating the stud, (d) disposing the stud into the powder within the bed to adhere the powder to the stud and (e) curing the powder on the stud.